There’s a number of operations it can perform on a new asset that can be very slow, especially if your file size is 300MB which is pretty big. If you don’t need them, turn off auto generate lightmap UV’s and auto generate collision. Especially if your mesh is complex and high poly those operations will be very slow and they’re on by default when you import. If you have your own lightmap UV’s set up for your mesh then you don’t need the auto option there, and if you don’t need collisions on the mesh or you have your own collision mesh then you don’t need that option either.